It is known from Flanagan et al. U.S. Pat. No. 5,159,595 issued Oct. 27, 1992 and entitled "Ring Transmission System" to provide an optical communication system in the form of a plurality of nodes coupled in a ring via two multiplexed communication paths providing for transmission in opposite directions around the ring. In normal operation, communications are effected between the nodes in both directions via the two paths. In the presence of a fault such as a fiber cut, this is detected in the two nodes immediately adjacent to the fault, and communications are maintained via both paths forming a folded loop, signals being coupled between the paths at these two nodes adjacent to the fault. Such systems have become known as bidirectional line switched ring (BLSR) systems, and typically serve for communicating SONET signals in which case they are commonly referred to as SONET ring systems.
BLSR systems provide effective fault protection, or survivability, for new communication system installations, but can not be easily applied to already-existing (synchronous or asynchronous) communication systems without requiring costly equipment upgrades. In addition, BLSR systems have disadvantages in that they have a high utilization of optical fibers, do not provide for 1:N (N&gt;1) protection (i.e. protection of N working (W) channels using one protection (P) channel), and they are not bit-rate or wavelength transparent (i.e. a change in wavelength or bit rate, such as a change from SONET OC-48 to OC-192 signals for increased capacity, involves a change in equipment). Furthermore, BLSR systems have the limitations that all nodes around the ring must be of the same type and must have the same capacity.
In order to provide survivability of existing communication systems without replacing them with ring systems, it is possible to use digital cross connects (DCCs) at the nodes of the system for rerouting signals in the event of a link failure. DCCs are electronic switches, for example operating on DS3 signals. However, the use of DCCs involves considerable disadvantages of cost, equipment capacity, complexity, size, and power consumption, and slow protection in the event of a fault.
It has also been proposed to use optical cross connects (OCCs) to provide for survivability of optical communication systems or networks, the OCCs serving to switch optical signals. However, it has been necessary for such OCCs to be large (for example, a 72.times.72 cross connect for an OC-48 optical signal carrying 16 DS3 signals) and for large numbers of such OCCs to be required in a network. Large OCCs have disadvantages of involving relatively new technology with low or unproved reliability and concerns of crosstalk and loss, as well as having a large size and cost.
It is also known from Wu U.S. Pat. No. 5,442,623 issued Aug. 15, 1995, entitled "Passive Protected Self Healing Ring Network", and from "A Novel Passive Protected SONET Bidirectional Self-Healing Ring Architecture" by Tsong-Ho Wu et al., IEEE Journal of Lightwave Technology, Vol. 10, No. 9, September 1992, to provide a ring network with a passive optical fiber protection ring. In such a network optical switches located at each node serve for protection switching of traffic to the protection ring in the event of a fault. While this can avoid some of the disadvantages of ring systems discussed above, it has disadvantages in that the optical fiber protection ring is unused in normal operation (i.e. in the absence of a fault), the traffic of the ring system in normal operation is conducted via the optical switches with a consequent decrease in the reliability of the system, and the complexity of the optical switching is increased in order to provide, as is desirable, for protection switching for span failures. In addition, this is a ring system which does not address survivability of existing communication systems using point-to-point communication links.
An object of this invention, therefore, is to provide a communication system which avoids or reduces the above disadvantages of the prior art.